In general, the cleaning with ultrasonic waves is well known. Ultrasonic cleaning is mainly being used in industrial piece-part production and employed especially in electrical engineering, in precision mechanics, in metal working, or also in the production of circuit boards. Other typical fields of application for this type of object cleaning can be found in photo industry as well as in medical technology. The use of ultrasonic waves serves to particularly remove dirt particles from objects, and is suggested when the impurities can only inadequately be reached and removed with mechanical cleaning devices such as, for example, brushes or polishing means.
For the ultrasonic cleaning according to methods known in the art as well as according to the method of the present invention it is necessary that the object to be cleaned is arranged within a liquid, so that the sound waves that are emitted from an ultrasonic generator using suitable ultrasonic oscillators can be transmitted via the liquid medium onto the object to be cleaned. So-called sound source fields emit the sound that impinges on the object to be cleaned.
Especially during a semiconductor production process different cleaning steps are necessary in order to remove dirt particles as well as particles that are formed during processing from the semiconductors, e.g. from wafers or disks. These particles that particularly originate from the mechanical processing of the semiconductor elements (substrates), such as e.g. from sawing, can be sawing particles that consist of the substrate's material. Also, a so-called “slurry” or abrasive powder emulsion is often used. This slurry comprises silicon carbide or aluminium oxide in a glycol or oil containing mixture. These impurities (motes, particles, remainders of organic compounds, etc.) must be removed in one or several cleaning steps in order to ensure the desired product characteristics.
Methods are known in the prior art in which the objects to be cleaned, such as for example wafers or disks, are first sorted into carriers. Subsequently, these are either manually or automatically submerged into tanks containing a suitable liquid, wherein the carriers are stationary fixed or, alternatively, moved within the cleaning tank either by a handling system or manually.
In order to achieve a better cleaning result, mostly several different tanks are provided, such as e.g. 10 to 20 tanks, which have different liquids and/or different sound sources. The sound sources themselves emit with different frequencies, so that in the individual tanks, different cleaning results can be achieved.
In order to support the cleaning effect the tanks are mostly operated warm, wherein the temperature is usually adjusted between 30° C. and 60° C. The frequencies lie in the ultrasonic range and are emitted by sound sources which are commonly located at the bottom as well as partly on the walls of the cleaning tanks. Alternatively or additionally, so-called submersion oscillators are used, which preferably are located at those places of the cleaning tank where one expects the realization of a uniform as possible distribution of the sound waves within the cleaning bath. According to a preferred alternative of the prior art the submersion oscillator(s) is/are provided as plate oscillator(s) and mostly placed below the objects to be cleaned. The plate oscillators are normally used for large-area parts in order to ensure that the sound waves reach the entire object.
The typical total time of such a cleaning process including drying approximately amounts to one hour and mostly requires several manual or mechanically assisted relocation steps, resulting in relatively high breaking rates. Thus, and in particular in view of the processing of future wafers with an edge length of 200 mm and a width of less than 200 μm, there is a need for alternative cleaning methods by which the objects to be cleaned can be treated quicker, cheaper, and more gently.
U.S. Pat. No. 4,979,994 discloses a method and an apparatus for the cleaning of printed circuit boards, where the printed circuit board is treated in a liquid by sound. In order to ensure that the cleaning also takes place in the space that is located between the electrical components and the printed circuit board, the orientation of the sound source fields is inclined, i.e. by an angle between −60 and +60 degrees to the vertical of the printed circuit board. These undercuts shall then be reached by reflection. The respective sound source field and the acoustically irradiated printed circuit board remain stationary with respect to each other.
DE-C 1 078 406 discloses an apparatus for the cleaning of metals by ultrasound. The device consists of a rotating drum that is funnel-shaped. A sound source field, whose position is adjustable along the circumferential surface of the drum, is arranged stationary.
DE 192 22 423 C2 discloses an apparatus for treating substrates, in particular semiconductor wafers. These substrates are mounted in a fluid tank on a bar-shaped carrier element. In order to achieve a better cleaning yield, the carrier element together with the substrate can be elevated and lowered, so that the distance between sound source field and substrate can optionally be larger or smaller.